In order to increase the efficiency and the performance of gas turbine engines so as to provide increased thrust-to-weight ratios, lower emissions and improved specific fuel consumption, engine turbines are tasked to operate at higher temperatures. As the higher temperatures reach and surpass the limits of the material comprising the components in the hot section of the engine and in particular the turbine section of the engine, new materials are desired.
As the engine operating temperatures have increased, new methods of cooling the high temperature alloys comprising the combustors and the turbine airfoils have been developed. For example, ceramic thermal barrier coatings (“TBCs”) were applied to the surfaces of components in the stream of the hot effluent gases of combustion to reduce the heat transfer rate and to provide thermal protection to the underlying metal and allow the component to withstand higher temperatures. These improvements helped to reduce the peak temperatures and thermal gradients. Cooling holes were also introduced to provide film cooling to improve thermal capability or protection. Also, ceramic matrix composites (“CMCs”) were developed as substitutes for the high temperature alloys. The CMCs in many cases provided an improved temperature capability and density advantage over the metals, making them the material of choice when higher operating temperatures were desired.
A number of techniques have been used in the past to manufacture turbine engine components, such as turbine blades, using CMCs. For example, silicon CMCs may be formed from fibrous material that is infiltrated with molten silicon. One such process is typically referred to as the Silcomp process. The fibers in this type of process generally have diameters of about 140 micrometers or greater, which prevents intricate, complex shapes, such as turbine blade components, to be manufactured by the Silcomp process.
Another technique of manufacturing CMC turbine blades is the method known as the slurry cast melt infiltration (“MI”) process. In one method of manufacturing using the slurry cast MI method, CMCs are produced by initially providing plies of balanced two-dimensional (2D) woven cloth comprising silicon carbide (SiC)-containing fibers, having two weave directions at substantially 90° angles to each other, with substantially the same number of fibers running in both directions of the weave. The term “silicon carbide-containing fiber” refers to a fiber having a composition that includes silicon carbide, and preferably is substantially silicon carbide. For instance, the fiber may have a silicon carbide core surrounded with carbon, or in the reverse, the fiber may have a carbon core surrounded by or encapsulated with silicon carbide.
Other techniques for forming CMC components includes polymer infiltration and pyrolysis (“PIP”). In this process silicon carbide fiber preforms are infiltrated with a preceramic polymer, such as polysilazane and then heat treated to form a SiC matrix.
Still another technique for forming CMC components may include an oxide/oxide process. In this type of processing, aluminum or alumino-silicate fibers may be prepregged and then laminated into a preselected geometry.
Components may also be fabricated from a carbon fiber reinforced silicon carbide matrix (C/SiC) CMC. The C/SiC processing includes a carbon fibrous preform layed up in the preselected geometry. As utilized in the slurry cast method for SiC/SiC, the tool is made up of graphite material. The fibrous preform is supported by the tooling during a chemical vapor infiltration process at about 1200° C., whereby the C/SiC CMC component is formed.
Current methods for forming CMC blades fail to permit the formation of an integral platform. Subsequent formation of the platform and/or the installation of metallic platform structures fail to provide the desired performance characteristics for the blade and may result in disengagement of the platform structure from the airfoil and loss of adequate sealing.
What is needed is a composite having an integral platform structure that is easily formed and provides the desired performance characteristics of a CMC blade.